1. Field of the Invention
The present invention relates to a sheet feeding device adapted for use in a computer, a copying machine, a printer, a facsimile apparatus, a word processor, a typewriter or the like, and more particularly to a sheet feeding device utilizing a travelling vibration wave for sheet feeding.
2. Related Background Art
A sheet feeding device utilizing travelling vibration wave is disclosed for example in the Japanese Laid-Open Patent Sho 59-177243. That sheet feeding device is provided with a pair of vibration member composed of elastic material for pinching therebetween a sheet with a suitable pressure, and it advances said pinched sheet by generating travelling vibration waves in said vibration members. Two groups of electromechanical energy conversion elements such as piezoelectric elements are adhered or maintained in contact with each vibration member in such a manner that the distance between said groups is equal to an odd multiple of .lambda./4, and, in each group, the piezoelectric element are arranged with a pitch of .lambda./2 and with alternating expansion-shrinkage characteristic. One of said groups of piezoelectric elements on each vibration member is given an AC electric field of a frequency close to the natural frequency of the vibration members (in practice said frequency is selected as the natural frequency of one of the vibration members) while the other group is given the same AC electric field with a phase difference of .pi./2 obtained by a .pi./2 phase shifter, thereby generating, in the vibration members, travelling vibration waves symmetrically to the feeding plane of the sheet. Said travelling vibration wave causes a kind of oval movement on a surface of the vibration member opposite to the surface where the piezoelectric elements are present. Thus mutually opposed points of the vibration members execute oval movement symmetrically to the feeding plane of the sheet, whereby the sheet is advanced by friction.
FIG. 4 schematically shows the principle of sheet feeding by the travelling vibration waves, wherein vibration members 1 and 5 support a sheet 9 with a suitable pressure, and travelling vibration waves are generated in said vibration members 1, 5. In this state, each particular point on the surface of the vibration member 1 or 5 moves along an elliptic trajectory. A point on the surface of the vibration member 1 moves clockwise along the elliptic trajectory in case the travelling vibration wave moves to right as indicated by a thick arrow. As the travelling vibration waves generated in the vibration members 1, 5 are designed to have a spatial phase difference of 180.degree., the peak portions, facing the sheet, of the travelling vibration waves of the vibration members proceed always in mutually opposed manner. Since the peak portions of the vibration members 1, 5 move in a direction opposite to the proceeding direction of the vibrations, there is generated a sheet feeding force in a direction indicated by a thin arrow. In the bottom portions of the vibrations, there is generated a sheet feeding force in a direction the same as said proceeding direction, but said force is smaller than in the peak portions because the lower pressure in such bottom portions results in a lower friction between the sheet and the vibration members. Consequently, the sheet feeding force in total appears in the direction opposite to the proceeding direction of the travelling vibration waves.
However such sheet feeding device, in which the travelling vibration waves are generated in the vibration members with a constant AC voltage, has been associated with a drawback of fluctuation in the sheet moving speed, because a fluctuation in the load resulting for example from unevenness in the sheet thickness varies the amplitude of the travelling vibration wave.